How to programatically add a product to the cart using a function?

Hello,

Sorry for the long delay.
To add a product with custom price programmatically you would have to do something like this:

// Sets product price
add_action( 'woocommerce_before_calculate_totals', function($cart_obj){
	if ( is_admin() && ! defined( 'DOING_AJAX' ) ) {
		return;
	}

	foreach ( $cart_obj->get_cart() as $key => $value ) {
		if ( $value['product_id'] == 6356 ) {
			$value['data']->set_price( 99 );
		}
	}
}, 10, 1 );

// Adds product to cart
add_action( 'wp', function () {
	if ( is_admin() && ! defined( 'DOING_AJAX' ) ) {
		return;
	}
	
	$product_id = 6356;

	foreach ( WC()->cart->get_cart() as $key => $val ) {
		$_product = $val['data'];
		if ( $product_id == $_product->get_id() ) {
			return true;
		}
	}

	WC()->cart->add_to_cart( $product_id );
} );